Active control of turbulence in boundary layer flows

Abstract

Preventing the transition to turbulence of a laminar flow and suppressing the variance of a turbulent flow, perhaps with the ultimate goal of inducing relaminarization, or alternatively controlling eddy fluxes produced by a given level of fluctuations are in themselves problems of great practical importance. In addition, understanding the physical mechanism of turbulence and turbulent transition should lead either to methods of control or to an explanation of why such control is not possible. From this perspective the control problem is seen as a test of physical theory. From the viewpoint of practical engineering, a comprehensive theory of the transition process and of the maintenance of fully developed turbulence that both implied new control mechanisms and provided a means of testing proposed mechanisms would be of great utility even if the result were only to discourage the search for e.g. a passive compliant membrane that relaminarized the turbulent boundary layer or an acoustic intervention to control turbulence in a free jet. Extensive attempts to reduce drag in turbulent boundary layer flow by imposing a variety of active and passive control measures have shown that in the absence of applicable theory it is very unlikely that an optimal method can be identified. Similar problems are faced if the aim is to control pressure fluctuations in order to reduce acoustic radiation or if we aim at increasing mixing in order to enhance chemical reactions in combustion problems.